1. Field of the Invention
The present invention relates to a hydrogen-occlusion-alloy electrode for use as a negative electrode of an alkaline storage battery such as a nickel-hydrogen storage battery, and more particularly, to a hydrogen-occlusion-alloy electrode capable of restraining the internal pressure of a battery from rising when the battery is overcharged, thereby prolonging a cycle life of the battery.
2. Description of the Related Art
A nickel-hydrogen storage battery is recently attracting attention as a high-capacity battery. The nickel-hydrogen storage battery has a negative electrode, which is a hydrogen-occlusion-alloy electrode comprising a hydrogen-occlusion-alloy supported on a current collector, and a positive electrode comprising a nickel hydroxide as a positive electrode active material supported on a current collector, and uses an alkaline solution as the electrolyte.
One of conventionally known hydrogen-occlusion-alloy electrodes for use in this type of storage battery is produced by mixing powder of a hydrogen-occlusion-alloy, which is capable of reversible occlusion and release of hydrogen, with powder of a binding agent, such as polytetrafluoroethylene powder, polyethylene powder or polypropylene powder, and then molding the mixture into a sheet. Another hydrogen-occlusion-alloy electrode known in the art is produced by applying or filling a hydrogen-occlusion-alloy to or in a current collector, which is an electrically conductive meshed sheet or perforated metal sheet having a desired rate of hole area, so that the current collector carries the hydrogen-occlusion-alloy thereon.
Of these two electrodes, the latter electrode is generally produced as described below.
First, a slurry containing hydrogen-occlusion-alloy powder is prepared in the following manner: A predetermined amount of hydrogen-occlusion-alloy powder having a predetermined particle diameter is dispersed in an aqueous solution of thickener, which is prepared by dissolving a predetermined amount of one or more thickeners such as methyl cellulose, carboxymethyl cellulose, polyethylene oxide and polyvinyl alcohol in ion-exchange water or distilled water. At this time, in order to increase the binding force between the particles of the hydrogen-occlusion-alloy powder to be supported on the current collector and thereby prevent the alloy powder from peeling off the collector, a suitable amount of binder powder, such as polytetrafluoroethylene powder, polyethylene powder, polypropylene powder and polyvinylidene fluoride powder, is added to the solution. Further, a suitable amount of conductive powder, such as cobalt powder, copper powder and carbon powder, is added to the solution, to increase the electrical conductivity of the hydrogen-occlusion-alloy so that the alloy supported on the current collector has enhanced current collecting capability as the negative electrode.
A current collector, which is a perforated nickel sheet or a nickel net, for example, is immersed in the slurry prepared as described above, and then is drawn up at a predetermined rate so that the slurry is filled in and supported on the current collector.
Subsequently, the slurry supported on the current collector is dried, and then the sheet or net with the dried slurry is rolled under a predetermined pressure to control the thickness of the dried slurry layer to a predetermined thickness and also to make the slurry layer closely adhere to the current collector, thereby obtaining a hydrogen-occlusion-alloy electrode.
In the case where polyvinylidene fluoride powder is used as the binding agent, after the above rolling step the whole structure is baked in a nitrogen atmosphere or in vacuum at a temperature of 150 to 210.degree. C., for example, to thereby soften the binding agent so that the binding agent is firmly combined together.
When a nickel-hydrogen storage battery is charged, the positive electrode thereof generates oxygen gas. The oxygen gas thus generated is reduced to water as it reacts with hydrogen occluded in the negative electrode.
Generally, however, a potential at which charging reaction of the hydrogen-occlusion-alloy takes place is close to an electrolytic potential of water, and accordingly, when the battery is overcharged, a large quantity of hydrogen gas is produced due to the electrolysis of water constituting the electrolyte. Consequently, the gas pressure of the hydrogen gas rises, increasing the internal pressure of the battery. In fact, the hydrogen gas accounts for about 90% of the gases exerting the internal pressure of the battery.
Usually, in the nickel-hydrogen storage battery, when the internal pressure exceeds 1.5 MPa, a safety valve built in the battery operates so as to release the internal gases. If this operation repeatedly takes place, however, the amount of the electrolyte gradually decreases, shortening the life of the battery.
Increase in the internal pressure can be controlled to some extent by increasing the capacity of the negative electrode. Increasing the capacity of the negative electrode, however, is not desirable in the light of the demand for batteries with a higher energy density.